1. Field of the Invention
The invention relates generally to digital communication systems. More particularly, this invention relates to a method and apparatus for echo cancellation in full duplex asymmetric communication systems.
2. Description of the Related Art
Communication systems are very common in today's society. These systems have evolved from devices that provide simple half-duplex data transmissions to sophisticated full-duplex systems providing voice, data, and video transmission and reception. Half-duplex communication commonly refers to the communication of information between a transmitter and a receiver in one direction at a time. On the other hand, full-duplex communication commonly refers to the communication of information between the transmitter and receiver in both directions at the same time, i.e., simultaneously. However, with sophistication comes added complexity which often necessitates corrective measures. One complexity that is associated with fill-duplex systems is a phenomenon known as “echo”. The production of echo in a full-duplex system is often attributed to leakage of at least a portion of a transmitted signal into an unintended receiver, such as a receiver portion of a transceiver or a co-located receiver.
FIG. 1 shows a functional block diagram of an exemplary modem system. At a near-end of the system 100a is a transmitter 110a and a receiver 120a. The transmitter 110a and receiver 120a are isolated or separated from each other by a hybrid 150a. As is well known in the art, a hybrid may be defined as a circuit that routes signals from one source (e.g., the transmitter 110a) to a desired output port, while preventing the signals from passage to an unintended destination (e.g., the receiver 120a). The transmitter 110a includes a digital modulator 112a that modulates information signals onto a carrier signal in preparation for transmission over a communication medium 170. The communication medium 170 may be a wired (e.g., telephone lines) and/or a wireless (e.g., airwaves) medium. The transmitter 110a further includes a digital-to-analog converter (DAC) 114a, which converts digital signals received from the digital modulator 112a into analog signals prior to transmission over the medium 170. The digital signal stream has a sampling rate of “fs”. Similarly, the receiver 120a includes an analog-to-digital converter (ADC) 124a, which converts analog signals received from the communication medium 170 into digital signals. The receiver 120a further includes a digital demodulator 122a that demodulates the digital signals from the carrier signal for further processing at an ultimate destination (not shown in this figure), e.g., a computer, television, or other application device.
The far-end portion of the system 100b comprises a mirrored-structure of the near-end portion of the system 100a. More particularly, the system 100b further comprises a transmitter 110b and a receiver 120b. The transmitter 110b and receiver 120b are isolated or separated from each other by a hybrid 150b. The transmitter 110b includes a digital modulator 112b that modulates information signals onto a carrier signal for transmission over a communication medium 170. The transmitter 110b further includes a digital-to-analog converter (DAC) 114b, which converts digital signals received from the digital modulator 112b into analog signals prior to transmission over the medium 170. Similarly, the receiver 120b includes an analog-to-digital converter (ADC) 124b, which converts analog signals received from the communication medium 170 into digital signals. The receiver 120b further includes a digital demodulator 122a that demodulates the digital signals from the carrier signal for further processing at an ultimate destination (not shown in this figure), e.g., a computer.
In practice, at least a portion of signals transmitted from the near-end transmitter 110a leak through the hybrid 150a into the near-end receiver 120a. This leakage contaminates the near-end receiver 120a in the form of an echo by mixing with or superimposing signals received from the far-end transmitter 110b. Thus, this superimposition causes interference by the echo signal with the intended information signals. The same is true for the far-end transceiver.
To circumvent such echo in full duplex systems, one of two methods may be applied. The first method is frequency division multiplexing (FDM), which may be defined as a multiplexing technique that uses different frequencies to combine multiple streams of signals for transmission over a communications medium. More particularly, forward and reverse streams of signals travelling in opposite directions occupy different portions of the frequency spectrum, with the effect that they can be easily separated in frequency at the receivers through a variety of signal processing techniques. FDM is not bandwidth efficient because it does not make full use of available bandwidth. The second method is echo cancellation, which allows forward and reverse signals to occupy overlapping frequency bands. A copy of the echo signal is reconstructed and subsequently subtracted at the affected receiver. Current echo cancellation techniques have been burdened in many applications by too much computational power and inferior speed performance rendering them undesirable.
Therefore, there is a need in the communications technology for a method and system that reduces computational requirement of echo cancellation. The method and system should be adaptable using common efficient and stable techniques without adding implementation hindrances.